Drive Strength and delay problem

[shivamgarg]

Hi Folks,

If i have a chain of inverters, each with a drive strength of X and another chain of inverters each with size of 10X. Which one should have more delay? or which should have an early response?

Thank You.

 

[veenanjaliallada]

cells having more drive strength can drive more number of cells (fanout).They are faster cells
hence inverter chain with 8X drive strength have lesser delay and inverter chain with 1X drivestrength has more delay.
to meet timing of your design 1X buffers need to be upsized if required.

 

[shivamgarg]

time delay depends on f= g h, where f= effort delay, g = logical effort and h = electrical effort. Since, the drive is same so g and h still remains constant. Shouldnt it depend on what load they are driving?

 

[praneethrajkanakam]

time delay depends on f= g h, where f= effort delay, g = logical effort and h = electrical effort. Since, the drive is same so g and h still remains constant. Shouldnt it depend on what load they are driving?

In this case it depends of the load the series of inverters driving.

Increase in Transistor width -> Increased Drive strength -> Increased Parasitic Capacitance -> Reduced Resistance

CASE 1 : Delay = RC; C=Cl+Cp
If Cp heavily dominates Cl; then larger width transistors results in increase in delay( due to increase in parasitic capacitance).

CASE 2 : If Cl dominates Cp; small increase in Cp will not contribute much to the total capcitance but at the same time reduces resistance. In this case using larger transistor reduces delay.

It is different for different scenarios.